/dev/urandom have their own, separate secondary pools that are fed off of a main pool when entropy is "depleted" in the second level pools. This is an area of research for us as well, since Linux's entropy estimation algorithm fails in situations where the timing deltas of entropy gathering events (IRQs and disk IOs) are actually predictable, so it's possible that the second level pools are not being refreshed at appropriate times.
If you write to
/dev/urandom, it goes into the primary pool by tradition. This is what the rc scripts do on bootup with the random seed file on disk.
BTW, it's absolutely the wrong solution to get entropy from another source on the network (for many reasons, but one is that you can't do a secure HTTPS handshake without, you guessed it, unguessable random numbers). The whole point here is that we are looking for a way for 500 Linux instances on EC2 to have different entropy pools before the kernel completes boot. The only possible solution is for the hypervisor (Xen for Amazon) to provide a simulated HW RNG that pulls entropy from a real HW RNG or from an entropy daemon in the hypervisor.
The best way to learn about Linux RNG basics is Gutterman et. al. Analysis of the Linux Random Number Generator
. Several of the issues they describe have been addressed, such as their PFS concerns, but their description of the entropy pools is still accurate.